Just to clarify, by "WATSAN in the health sector" do you mean in health settings (e.g. clinics) - which Florence Nightingale looked at in 1860, or wider impacts on health?
If the latter, then these might help a bit (but not a lot).
CAIRNCROSS, S. and KOLSKY, P., n.d. Environmental Health and The Poor: Our shared responsibility [online]. Loughborough University, UK. Available from: https://www.lboro.ac.uk/orgs/well/resources/Publications/Advocacy7 document - Front cover + insides.pdf.
CAIRNCROSS, S., O’NEILL, D., MCCOY, A., and SETHI, D., 2003. Health, Environment and the Burden of Disease; A guidance note [online]. Available from: https://www.lboro.ac.uk/orgs/well/resources/Publications/DFID7 Health.pdf.
The R2HC literature review a few years ago only found 4 (?) papers on WASH and health that met their narrow criteria.
BLANCHET, K., ROBERTS, B., SISTENICH, V., RAMESH, A., FRISON, S., WARREN, E., SMITH, J., HOSSAIN, M., KNIGHT, A., LEWIS, C., POST, N., WOODWARD, A., RUBY, A., DAHAB, M., PANTULIANO, S., and PARTNERS, B.R., 2015. An evidence review of research on health interventions in humanitarian crises.
However this is a caveat:
LOEVINSOHN, M., MEHTA, L., CUMING, K., NICOL, A., CUMMING, O., and ENSINK, J.H.J., 2015. The cost of a knowledge silo: A systematic re-review of water, sanitation and hygiene interventions. Health Policy and Planning [online]. 30 (5), pp. 660–674. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4421832/8.
The Neglected Tropical Diseases are mostly preventable, but by non "health" measures, such as solid waste management. Even cholera can be prevented but the search for a vaccine continues. Even with a vaccine, people need clean water and adequate sanitation. There is a difference between "public health" - vaccines, anti-smoking etc., which is managed by a Health Ministry and "environmental health" where the lead is by water, agriculture, transport or industrial organisations and which has an aspect of protecting health but is not their primary role. Allocating health funds to road safety improvements may be more efficient in reducing the human costs of road accidents but can you see a health ministry funding that?
There are issues of what do health professionals consider evidence? The rigor required for research can lead to a bias away from more relevant research. Testing a household water treatment device is much easier than a comparative study on a reticulated water supply. Even trying to define safe water or adequate sanitation is difficult, making research problematical. What is fine for one person may not be acceptable for her neighbour - unlike, say, mass vaccination or using an aspirin. Some areas have not even been researched - take the impact of urban drainage on cholera for example, which I think there has only been one study on, yet was observed back in 1860.
I did chat to a lecturer at a local medical school. Even though the impact of sanction is recognised (FERRIMAN, A., 2007. BMJ readers choose the sanitary revolution as greatest medical advance since 1840. BMJ : British Medical Journal. 334, p. 111.), it is not part of the curriculum, with only a mention at the start of their course.
I would suggest an alternative approach. Lock all the toilets in your building and turn off the water. Personal experience may be more powerful than yet another research project.